1. Field of the Invention
This invention relates to materials particularly useful in the manufacture of electrochemical cell electrodes.
2. Description of the Prior Art
Electrodes for use in electrochemical cells, such as fuel cells, are well known in the art. One common type of electrode is the gas diffusion electrode. A pair of gas diffusion electrodes are disposed on either side of a compartment containing a matrix which is soaked with electrolyte. A catalyst is disposed on the electrolyte facing surface of each electrode. Hydrogen is fed to the other side of one electrode while oxygen or air is fed to the other side of the other electrode. The gasses enter the electrodes and react with the electrolyte in the presence of the catalyst.
One type of gas diffusion electrode comprises a layer of polytetrafluoroethylene (PTFE) particles combined with a catalyst supported on carbon particles (i.e. catalyzed carbon), said layer being disposed on the surface of a porous carbon substrate (e.g. carbon paper). The PTFE prevents the electrolyte from filling up the electrode to such an extent that sufficient fuel or oxident cannot reach the catalyst. A gas diffusion electrode of this type is described in column 5 of commonly owned U.S. Pat. No. 3,857,737 to Kemp et al. The electrode may also be made by applying, to the surface of the carbon paper, a layer of uncatalyzed carbon particles combined with PTFE particles. Unsupported catalyst is then applied to this carbon/PTFE layer in a subsequent step. An electrode of this type is described in commonly owned U.S. Pat. No. 3,972,735 to Breault.
The most common methods for applying the carbon/PTFE layer (using either catalyzed or uncatalyzed carbon) are wet application methods. For example, carbon powder (catalyzed or uncatalyzed) and an aqueous PTFE dispersion are blended into an alcohol/water solution to form a co-suspension of carbon and PTFE therein. The suspension is caused to floc, such as by heating or adding a floccing agent. Floccing is the coalescence of the catalyzed or uncatalyzed carbon particles with the PTFE particles. A proper floc is one which achieves uniform size agglomerates of catalyzed or uncatalyzed carbon and PTFE particles and a homogeneous distribution or mix of the particles. In the direct filtration method the floc suspension is applied to the surface of a carbon paper electrode substrate which acts as a filter. Liquids pass through the substrate and a layer of the desired solids mixture (i.e. the floc) remains on the surface of the substrate. The article is then dried, compacted, and sintered. If the applied layer does not include a catalyst, a separate catalyst application step is then required.
A related technique is the indirect filtration (or filter transfer) method wherein the floc suspension is filtered through special filter paper instead of onto the carbon paper substrate. The wet floc is then transferred from the filter paper onto the carbon paper substrate, and is dried, compacted and sintered as in the direct filtration method.
Another technique for applying either the catalyzed carbon/PTFE layer or the uncatalyzed carbon/PTFE layer to an electrode substrate, called the dry floc method, is described in commonly owned U.S. Pat. Application Ser. No. 920,036 titled DRY FLOC METHOD FOR MAKING AN ELECTROCHEMICAL CELL ELECTRODE, Goller et al, filed on June 28, 1978, which is incorporated herein by reference. In that method carbon powder (catalyzed or uncatalyzed) and an aqueous PTFE dispersion are blended into an alcohol/water solution to form a co-suspension of carbon and PTFE therein. The suspension is caused to floc, such as by heating or adding a floccing agent. (If the carbon particles are catalyzed with platinum a small amount of phosphoric acid may be added at this point to inhibit burning of the catalyst layer during sintering of the electrode.) The excess liquid is removed by filtration and the wet floc is then dried and pulverized to a fine powder. The dry floc powder is applied to the surface of an electrode substrate by dispersing the powder as a cloud in a chamber over the substrate and pulling the powder onto the substrate by drawing a vacuum under the substrate. The electrode is subsequently compacted and sintered.
In practicing this dry floc method for making electrodes several problems have been experienced in the handling and storage of the powdered dry floc material, particularly in efforts to use the dry floc powder in a completely automated electrode manufacturing procedure. For example, the dry floc powder tends to agglomerate to larger particle size due to the PTFE components of the floc. This problem is compounded if phosphoric acid is added during the formulation of the floc, since the acid causes the dry floc powder to absorb moisture and the moist floc is tacky. That is one reason why the above mentioned co-pending Goller et al application recommends that the dry floc powder be introduced into the cloud chamber directly from the grinding apparatus. If the powder is simply collected from the grinding apparatus and stored, it would most likely require regrinding. In any case, the dry floc powder is quite difficult to work with, particularly in automated equipment, since the powder does not flow freely from hoppers and tends to stick to the wall surfaces of automated materials handling equipment, such as automatic weighing apparatus. Storing irregular shaped chunks of dry floc which are broken from a large sheet is also not satisfactory because the pieces tend to absorb excessive moisture (due to the presence of acid) during storage and cannot be ground properly because of this excess moisture. Automatic weighing of irregular shapes is also not very precise. Cutting a flat sheet of dry floc into small pieces results in densified material along the cut lines which ultimately adversely affects the properties of the applied dry floc layer.